In an HVACR system, conditioned air is supplied to each comfort zone in a building in proportional to the demand in that zone. Stages of temperature conditioning are energized as required to meet the total demand on the system.
An HVACR system often includes an outdoor air economizer for admitting fresh air into the building, thereby reducing the need for mechanical cooling. Depending upon the enthalpy of the outdoor air, a damper assembly on the economizer may be modulated between minimum and maximum positions to control the proportion of outdoor air used for cooling the building interior spaces. If an enthalpy sensor indicates that outdoor air temperature/humidity is excessive, the economizer damper is kept at its minimum position. In this minimum position, the HVACR system is taking in the minimum amount of fresh air judged to be necessary to maintain an adequate level of indoor air quality.
Staging controls for HVACR systems typically attempt to satisfy the cooling demand first with the outdoor air economizer, and if that is inadequate, thereafter by energizing successive stages of mechanical cooling. Of course, if the outdoor ambient air enthalpy is less than the enthalpy of the return air, it may be possible that outdoor air can be used to cool the building without energizing any mechanical refrigerant cooling stages. Conversely, if the outdoor ambient air enthalpy is greater than the return air enthalpy, it is unlikely that the cooling demand can be met entirely with air supplied by the economizer.
In the past, the economizer control has not been fully integrated with the temperature change conditions that are actually occurring in the zone of the building that is being cooled. There are many ways to determine when to economize and when to disable the compressor. Previously the decision to use the economizer and disable the compressor has not been based on the temperature in the zone. Typically, the decision as to whether or not to use the compressor has been made by comparison of the temperature of the outdoor air with a fixed reference temperature.
In cooling a commercial building that is occupied only during working hours, the temperature setpoint for a given zone is usually set to a relatively high level during periods in which the building is not occupied. The setpoint during night hours and on weekends is generally at a level that would be perceived as being uncomfortably warm by occupants of the building. In the morning on a day in which the building is going to be occupied, the setpoint is typically automatically adjusted downward. At this time, the HVACR system sees a marked increase in the cooling load in the zone. The control system of the HVACR system attempts to provide adequate cooling to the zone to decrease the temperature in the zone to approximate the daytime setpoint. Typically, when faced with a large setpoint change, the control system commands the compressors to energize and mechanically cool the air in the zone, without regard to whether adequate cooling capacity is available by using the economizer only.
U.S. Pat. No. 4,379,484 to Lom et al. discloses a control system that is typical of the control systems in the industry today. While the idea provides for control of the economizer, this control is limited by the fact that the controlling signal is proportional to the deviation of the discharge air temperature from the setpoint value of the discharge air. The temperature of the discharge air, of course, is not indicative of the temperature or temperatures changes in the zone that is being cooled.
U.S. Pat. No. 4,404,815 to Gilson discloses an economizer control system. This control system senses the temperature of the mixed return air from the cooled zone and the outdoor ambient air from the economizer just prior to that mixed air entering the indoor coil of the air conditioning unit. The control system uses this temperature to select the combination of economizer and compressor cooling necessary to meet the cooling requirements. Again, this temperature has a temperature that is not truly indicative of the temperature and temperature changes occurring in the zone that is to be cooled.
U.S. Pat. No. 4,389,853 to Hile discloses a third means of controlling an economizer. The idea disclosed in this patent utilizes a thermostat to sense the temperature of the air in the zone to be cooled and compares that temperature to a setpoint. The thermostat is designed to generate an increasing cooling command signal as the variance between the setpoint and the desired temperature increases. As the command increases, greater reliance is placed on mechanical cooling as distinct from economizer cooling. Accordingly, this system will command mechanical cooling during periods of large setpoint variance without regard to whether or not the economizer can adequately provide the desired cooling necessary.
Economizer cooling is the most energy efficient means of cooling a zone. Electrical energy is being sent only to power the fans necessary to force the air through the zone. Mechanical cooling also requires these fans plus requires the substantial energy necessary to run the compressors and related equipment. Accordingly, it is always desirable to cool a zone utilizing the economizer when the economizer has the capability of providing the desired level of cooling. In order to most effectively determine if the economizer is capable of providing the desired level of cooling, it is necessary to integrate the control of the economizer with the actual temperature and temperature changes that are occurring in the zone to be cooled. It would be a decided advantage in the industry to have a economizer control system that is fully integrated into temperature and temperature changes that are occurring in the zone to be cooled. Such a system would make maximum use of the cooling that is available from the outside ambient air and would minimize the mechanical cooling that is required to maintain a zone at a desired setpoint. This enhanced means of control would be especially useful during the times of large setpoint change, as when the setpoint in the zone is changed from the relatively high night time setpoint to the relatively lower day time setpoint.